Saliva plays a crucial role in the first stage of digestion — oral digestion in the mouth (buccal cavity). It is produced by three pairs of salivary glands: the parotid glands (largest, in front of the ears), the submandibular glands (below the jaw), and the sublingual glands (under the tongue). About 1.0–1.5 litres of saliva are secreted every day. The key enzyme in saliva is salivary amylase (also called ptyalin), which begins the chemical digestion of carbohydrates (starch) by converting starch into maltose. Saliva also lubricates food and has antibacterial properties.
Saliva is produced by three pairs of glands: parotid, submandibular, and sublingual.
Daily saliva production: 1.0–1.5 litres.
Key enzyme: salivary amylase (ptyalin) — digests starch into maltose.
Optimal pH for salivary amylase: 6.8–7.0 (neutral to slightly acidic).
Salivary amylase is denatured by stomach acid (pH 1.5–2.0) — digestion stops in the stomach.
Mucin in saliva lubricates food and forms the bolus for swallowing.
Lysozyme in saliva kills bacteria — antibacterial function.
Saliva is 99.5% water.
Digestion of starch starts in the mouth (oral cavity) — the only site of carbohydrate digestion before the small intestine.
The most important digestive role of saliva is through the enzyme salivary amylase (ptyalin):
Reaction: Starch → (salivary amylase) → Maltose + Dextrins
Key details: • Salivary amylase is a carbohydrase — it breaks down carbohydrates • Optimal pH: 6.8–7.0 (slightly acidic to neutral — the pH of saliva) • Temperature: works best at body temperature (37°C) • Digestion stops in the stomach: salivary amylase is denatured (inactivated) by the highly acidic gastric juice (pH 1.5–2.0) in the stomach
This means starch digestion in the mouth is partial — only about 30% of starch is digested by salivary amylase before it reaches the stomach. The rest is digested in the small intestine by pancreatic amylase.
Saliva performs a crucial mechanical role in digestion:
Moistening food: • Dry food particles are bound together and softened by saliva • Makes chewing (mastication) more efficient
Forming the bolus: • Saliva contains mucin — a sticky glycoprotein • Mucin coats food particles and binds them into a soft, slippery mass called the bolus • The bolus is the rounded lump of food that is swallowed
Facilitating swallowing: • The lubricated bolus slides easily down the oesophagus (food pipe) • Without saliva, swallowing would be difficult and painful
Dissolving taste chemicals: • Chemicals in food must be dissolved to stimulate taste receptors (taste buds) on the tongue • Saliva dissolves these chemicals, allowing us to taste food
Saliva has several additional roles beyond digestion:
Antibacterial action: • Saliva contains lysozyme — an enzyme that kills bacteria by breaking down their cell walls • Also contains immunoglobulin A (IgA) — an antibody that provides immune protection in the mouth • Helps prevent tooth decay and gum infections
Dental protection (buffering): • Saliva contains bicarbonate ions (HCO₃⁻) that neutralise acid produced by mouth bacteria • Prevents tooth enamel erosion (dental caries) • Remineralises early tooth decay with calcium and phosphate ions
Keeping mouth moist: • Prevents drying of mucous membranes in the mouth and throat • Dry mouth (xerostomia) causes difficulty in speaking, chewing, and swallowing
Speech: • Adequate moisture is necessary for clear articulation • Dry mouth can affect speech
Saliva is approximately 99.5% water, with the remaining 0.5% comprising:
Component | Function Salivary amylase (ptyalin) | Digests starch → maltose Mucin (mucus) | Lubricates food, forms bolus Lysozyme | Antibacterial — kills bacteria Immunoglobulin A (IgA) | Immune defence Bicarbonate (HCO₃⁻) | Buffers acid, protects teeth Calcium and phosphate ions | Remineralise teeth Water | Dissolves food chemicals, moistens
Saliva glands: • Parotid glands: produce mostly watery (serous) saliva, rich in amylase • Submandibular glands: produce mixed serous and mucous saliva • Sublingual glands: produce mostly mucous saliva (thick, lubricating)
pH of saliva: 6.8–7.0 (slightly acidic to neutral)
Saliva plays two main roles in digestion: (1) Chemical digestion: salivary amylase (ptyalin) breaks down starch into maltose. (2) Mechanical digestion: mucin in saliva lubricates food, binding it into a soft bolus that can be easily swallowed. Saliva also dissolves taste chemicals, has antibacterial properties (lysozyme), and protects teeth (bicarbonate buffers).
Saliva contains salivary amylase (also called ptyalin). It is an enzyme that breaks down starch (a complex carbohydrate) into maltose (a simpler sugar). It works at pH 6.8–7.0 (the neutral pH of saliva). Salivary amylase is denatured when it reaches the stomach (pH 1.5–2.0), so starch digestion by this enzyme is limited to the mouth and oesophagus.
The pH of saliva is 6.8–7.0 (slightly acidic to neutral). This is the optimal pH for salivary amylase to function. The bicarbonate ions in saliva also help maintain this pH and neutralise acids produced by oral bacteria, protecting teeth from acid erosion. When saliva reaches the stomach (pH 1.5–2.0), salivary amylase is denatured and digestion of starch stops.
Mucin is a glycoprotein (sticky protein) in saliva that coats and binds food particles together into a soft, smooth lump called the bolus. The bolus is then pushed to the back of the throat and swallowed. Mucin lubricates the bolus, making it easy to slide down the oesophagus. Without mucin, swallowing would be difficult.
Digestion of starch begins in the mouth (oral cavity/buccal cavity), where salivary amylase (ptyalin) converts starch into maltose. This is the only site in the human body where carbohydrate digestion starts. Starch digestion continues in the small intestine by pancreatic amylase. There is no carbohydrate digestion in the stomach.
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